Neutron formation evaluation tools interrogate the formation surrounding the borehole with high energy neutrons produced by a neutron source associated with the tool. Through various types of interactions by the neutrons with elements of the borehole and formation, the energy of some of the neutrons is reduced. Neutron detectors may detect (or “count”) neutrons received back from the formation having a particular energy level, for example thermal neutrons.
Neutrons are sensitive to the presence of hydrogen atoms, which are effective at reducing the energy of neutrons to the thermal level. Hydrogen atoms are indicative of formation properties such as porosity, which can thus be determined based on the number of arrivals of thermal neutrons at the neutron detector(s). In particular, a large amount of hydrogen atoms results in more thermal neutrons, which are then detected by the neutron detector. However, the presence in the borehole or formation of certain ions such as halogens, which have a tendency to capture thermal neutrons, reduces the count of neutrons received at the neutron detector. This negatively impacts the accuracy of a porosity calculation based on the number neutrons detected by the neutron detector.